The present invention relates to a method for determining an ophthalmic lens for a wearer. The method can be applied interchangeably to a unifocal or multifocal prescription. It also applies to microstructured lenses (pixelated lenses, diffractive lenses, Fresnel lenses and so on), adaptive lenses, graded index lenses and more generally any type of ophthalmic lens.
The invention also extends to the method of calculating the parameters for trimming and manufacturing an ophthalmic lens obtained by the method of determination.
A wearer can be prescribed a power correction, which is positive or negative (hyperopic or myopic wearer); the lens used for this type of prescription is a spherical lens or an aspheric lens. An astigmatic wearer has, in a plane perpendicular to the direction of gaze, a prescription for power that is different along different axes; the prescription is usually expressed as a prescription for a first power value corresponding to power along a main axis and a second power value along an axis perpendicular to the main axis. The lens used for this type of prescription is a tonic or atoric lens. Below, we shall call the proposed correction for such wearers a unifocal prescription.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, because of difficulties of accommodation in conditions of near vision. The prescription thus comprises a power value for far vision and a power addition representative of the power increment between far vision and near vision. Ophthalmic lenses that compensate for presbyopia are multifocal lenses, the most appropriate being progressive multifocal lenses, for which power varies continuously. Also known are bifocal or trifocal lenses, with breaks in continuity on the surface of the lens. Below, we shall call the proposed correction for such wearers a multifocal prescription.
Calculating the front and/or rear faces of multifocal and of unifocal lenses using optimization is known. For example, WO-A-98/12,590 discloses a method for determination by optimization of a set of multifocal ophthalmic lenses. This document proposes defining the set of lenses by considering the optical characteristics of the lenses, and notably power and oblique astigmatism under wearing conditions. The lens is optimized using ray tracing, from an ergorama associating a target object point with each direction of gaze under wearing conditions.
A method is also known from EP-A-0,990,939 for determining by optimization an ophthalmic lens for a wearer having an astigmatism prescription. This document proposes selecting a reference lens and using a ray tracing method and minimizing the difference between residual astigmatism and the astigmatism of the reference lens. The residual astigmatism is defined therein as the difference in amplitude and axis of astigmatism between prescribed astigmatism and the astigmatism generated by the lens. This method allows a better adaptation of lenses to astigmatic lens wearers, avoiding optical aberrations induced by the addition of a tonic surface. The calculation is performed in a reference frame linked to the eye, which takes into account the twisting effect on the eye when the wearer looks in a direction that is off-center.
In addition, in recent years, it has been made looked for customizing progressive ophthalmic lenses in order to better meet the needs of each wearer. WO-A-2007/068,819 teaches a method of determining a set of progressive ophthalmic lenses for a given wearer for whom a power addition has been prescribed in conditions of near vision, the method comprising a step consisting in measuring individual physiological parameters of the wearer in conditions of near vision. The method also includes a step of determining an ergorama which associates, on each lens, a point aimed at in each direction of gaze under wearing conditions and a step of determining a target power defect and a target for resulting astigmatism for each gaze direction under wearing conditions, the target power defect and resulting target astigmatism being determined from the measured physiological parameters for the wearer. The method further includes calculating the power required on each lens for the said ergorama by successive iterations to reach the target power defect and target astigmatism defect for each direction of gaze.
There is also known from WO-A-2007/068,818 a method for tailoring the progression length of as lens of progress.
There are also documents which teach measurement of physiological parameters and notably the position of the center of rotation of the eye. Thus, WO-A-2008/132,356 discloses a method for determining the position of the center of rotation of the eye.
U.S. Pat. No. 6,637,880 discloses a method for ray tracing and optimization of a lens, taking into account the distance between as reference point on the rear surface of the lens and the center of rotation of the eye of a wearer. This distance is obtained by adding, firstly, the distance between the reference point on the rear surface and the vertex of the cornea, and, secondly, the distance between the vertex of the cornea and the center of rotation of the eye. The distance between the reference point on the rear surface and the vertex of the cornea is calculated from data relating to the selected frame; this document proposes only considering the shape of the wearer's head, the lens data, characteristics of the frame, and wearing conditions, without providing details regarding calculation. The distance between the vertex of the cornea and the center of rotation of the eye is obtained by measuring the depth of the eye and application of a statistical law, establishing a relation between the depth of the eye and the distance between the vertex of the cornea and the center of rotation of the eye. In this document, the position of the center of rotation of the eye taken into account is consequently not the actual position. This results in the lens obtained by optimisation not perfectly satisfying the wearer.